1. Field of the Invention
The present invention relates to ceramic materials for use in enclosing a hollow tubular ceramic material by insert-casting with a melt of a metal such as aluminum or cast iron. Particularly, the invention relates to heat-insulating ceramic materials for use in insert-casting as port liners, exhaust manifold liners, piston cavities, cylinder top plates, etc. in gasoline engines, diesel engines and the like. The invention also relates to processes for producing such ceramic materials.
2. Related Art Statement
Recently, environmental pollution caused by with automobile exhaust gases has become a serious social problem, and methods for removing poisonous materials from the automobile exhaust gases with use of catalysts are mainly employed. Reduction in use amounts of noble metals such as Pt, Rh, etc. used as catalysts therefor results in a problem from the stand point of natural resources and costs. Further, four valve engines, which have recently increasingly been employed, pose a problem in that a catalytic purification performance is reduced by reduction in temperatures of exhaust gases.
As one method for solving the above problems, Japanese Patent Application Laid-open No. 63-236,759 proposes that the inner surface of an exhaust port or an exhaust port liner is lined with a port liner made of a ceramic material having a low Young's modulus so that the temperature of exhaust gases may be increased by its heat insulating property. Further, Japanese Patent Publication No. 57-3,629 and Japanese Patent Application Laid-open No. 56-92,164 disclose the addition of a rare earth element to aluminum titanate as also intended in the present invention.
However, there are problems that the materials disclosed in Japanese Patent Application Laid-open No. 63-236,759 suffer deterioration in strength due to a cooling-heating cycle owing to changes in temperatures of the exhaust gases when in use, and that the materials are broken.
In the inventions of both Japanese Patent Publication No. 57-3,629 and Japanese Patent Application Laid-open No. 56-92,164, dense sintered bodies having a low thermal expansion are obtained by adding a given amount of an oxide of Y, La and/or Ce to aluminum titanate. However, Young's modulus of all these sintered bodies are high, and they cannot be used for materials in insert-casting.
As the ceramic materials used for increasing the temperature by their heat insulating property, those having lower Young's modulus are preferred, because they can mitigate great compression stresses occurring through shrinkage of a molten metal during insert-casting of liners. NGK Insulators, Ltd. have formerly developed ceramic materials for use in insert-casting, in which not less than 65% by volume of aluminum titanate is contained as a crystalline phase, and proposed them in Japanese Patent Application Laid-open No. 63-236,759.
However, succeeding researches revealed that although the above materials had the Young's modulus E falling in the range from 50 to 2,000 kgf/mm.sup.2, some of them were broken upon insert-casting, while others were not. Therefore, stable insert-castability cannot be assured, which poses a practical problem.
The present invention is to solve the conventional problems mentioned above, and has been accomplished to provide practical ceramic materials suitable for insert-casting, which have excellent heat resistance, thermal shock resistance and heat insulation, are free from the occurrence of cracks owing to compression forces generated on insert-casting, and have durability in cooling-heating cycles during use. The invention also provides processes for producing such ceramic materials.